The present invention pertains to a process for producing free-flowing, non-agglomerating particles of a polyalkylene carbonate.
Thermoplastic polymers are commonly processed into many different physical forms such as particles, extrudates, sheets, pellets, and the like. The specific physical form selected will be determined by the ultimate end use and the properties of the polymer. Although certain thermoplastic polymers may be obtained in the desired shape and form directly from the polymerization reaction (as in suspension polymerization, for example), most must be re-processed into a useful physical form following polymerization.
Polyalkylene carbonates may be prepared by the copolymerization of carbon dioxide and one or more alkylene oxides. These polymers, which are comprised of alternating carbon dioxide and alkylene oxide monomer units, are generally obtained in the form of slabs following separation of the polymer from the solvent and catalyst used in the polymerization process. Although the slabs can be ground into a form acceptable for many applications, it may be desirable to convert the slabs into other physical shapes which are better suited for use in specific applications.
Polyalkylene carbonates have been employed in the preparation of molded foam patterns for use in lost foam casting (U.S. Pat. Nos. 4,773,466 and 4,633,929). One of the primary advantages of using a polyalkylene carbonate foam pattern in evaporative casting is that the polymer decomposes smoothly and without violent evolution of gas when it comes into contact with the molten metal during the casting process. The castings thus obtained have smooth surfaces with no significant defects due to carbon deposits.
To form the molded foam pattern, small particles of the polyalkylene carbonate can be suspended in water, impregnated with a volatile blowing agent, pre-expanded by exposure to steam, and then molded into the desired shape by further heating. For this application, it has been found that the polyalkylene carbonate particles should be relatively dense in order that the blowing agent be substantially retained after impregnation. It is also highly desirable that the particles be free-flowing, non-agglomerating, and substantially spherical in shape. These characteristics result in easier handling during the process used to form the molded foam pattern and also result in higher quality patterns. Furthermore, the particles should be substantially free of any unreacted alkylene oxide monomer, cyclic carbonate, low molecular weight oligomers, and solvent as these impurities will act as plasticizers and lower the glass transition temperature. The polyalkylene carbonate should have a glass transition temperature significantly above room temperature so that the discrete particles do not soften and agglomerate during handling and so that the molded foam patterns retain their shape in a warm environment. This is particularly critical when the polyalkylene carbonate employed is polypropylene carbonate, which has a glass transition temperature of only about 42.degree. C. even when highly pure.
Solvent/non-solvent precipitation is a technique commonly employed to isolate a polymer in solid form from a solution.
U.S. Pat. Nos. 3,953,383, 4,166,898, 3,900,424, and 4,500,704 teach to dissolve polyalkylene carbonates in certain solvents and to precipitate the polymers by adding the polymer solutions to certain non-solvents. The preparation of discrete, non-agglomerating particles is not taught by these references; descriptions of the physical form of the precipitated polymers obtained are not given. The specific solvent/non-solvent pairs taught are limited to chloroform/acetone, methylene chloride/acetone, tetrahydrofuran/water, dioxane/dilute aqueous hydrochloric acid, and tetrahydrofuran/dilute aqueous hydrochloric acid when the polyalkylene carbonate is an ethylene oxide/carbon dioxide copolymer and to chloroform/methanol, methylene chloride/methanol, tetrahydrofuran/methanol, tetrahydrofuran/aqueous acidic methanol, and benzene/methanol when the polyalkylene carbonate is a copolymer of carbon dioxide and an epoxide other than ethylene oxide.
U.S. Pat. Nos. 4,066,630 and 4,145,525 teach the precipitation of end-capped polyethylene carbonate and polypropylene carbonate from a chloroform solution using methanol as a non-solvent. Again, the physical form of the precipitate is not described and there is no suggestion that discrete, free-flowing particles from 30 to 3000 microns in diameter may be obtained by this method.
U.S. Pat. No. 3,706,713 teaches the precipitation of polyalkylene carbonates from a toluene+heptane solution using methanol as a non-solvent. The precipitate thus obtained is described as a white, free-flowing powder when the alkylene oxide in the polyalkylene carbonate is cyclohexene oxide.